1. Field of the Invention
Embodiments of the present invention relate to a device for optical coherence tomography for determining geometric structures and optical biometry of the eye. In particular, embodiments of the present invention relate to a device for optical coherence tomography for determining the power of an intraocular lens implant and the condition of the retina.
2. Description of Related Art
Modern intraocular lens (IOL) calculation formulas rely on the measurement of several geometric parameters of the eye in order to calculate the power of an implanted intraocular lens after cataract surgery. Third-generation formulas such as the SRK/T (Retzlaff J A, Sanders D R, Kraff M C, “Development of the SRK/T intraocular lens implant power calculation formula” J Cataract Refract Surg 1990; 16:333-340), Holladay 1 (Holladay J, et al., “A 3-part system for refining intraocular lens power calculations” J Cataract Refract Surg 1988; 14:17-24), and HofferQ (Hoffer K J., “The Hoffer Q formula: A comparison of theoretic and regression formulas” J Cataract Refract Surg 1993; 19:700-712.) require measurement of axial eye length and corneal curvature. Fourth-generation formulas such as Holladay 2 (available from Holladay et al., http://www.hicsoap.com/topic/12-hicsoap-professional-edition.aspx) and Haigis (Haigis W, et al., “Comparison of ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis” Graefe's Arch Clin Exp Ophthalmol 2000; 238; 765-773) also require additional parameters, including anterior chamber depth and lens thickness, which allow for more accurate calculations. These parameters are important for estimating the effective lens position (ELP) of the IOL in the eye.
An ultrasound A-scan may be used to measure axial eye length. However, the results from ultrasonic measurements may be less accurate and less reproducible than optical methods. First, the ultrasonic wavelength is typically longer than the optical wavelength making the ultrasonic measurements less precise. Second, if a contact ultrasonic probe is used, pressure applied to the cornea may distort the axial length measurement. Third, an immersion ultrasonic probe can be uncomfortable for the patient. Finally, it is difficult to accurately place the ultrasound probe on the desired measurement axis of the eye. However, the ultrasonic measurement can often be performed even when there is a dense cataract, which renders measurement using optical methods difficult.
The health of the eye is an important consideration prior to cataract surgery. Current optical biometers only obtain measurements on a single axis of the eye and are therefore unable to create an image of the cornea or retina.
Conventionally, measurements of corneal curvature and axial length are obtained with two different instruments or two different measurement beams. For example, if ultrasound is used to measure the axial eye length, a separate optical keratometer or a topographer are used to measure the corneal curvature. Some devices use optical methods to measure the axial length and corneal curvature. In these methods, partial coherence interferometery is used to measure the axial length and a separate optical keratometer is used to measure the corneal curvature. In other methods, the principle of partial coherence interferometry is used to measure the axial length, but a Scheimpflug image (Scheimplfug T., “Der Photoperspektograph and Seine Anwendung. Photogr” Korresp 1906; 43:516) is used to derive the anterior chamber depth and the lens thickness needed for the IOL calculation. The Scheimpflug principle describes how the image or camera plane, lens plane, and object planes of an optical system can be oriented such that the object plane is completely in focus at any depth.
The traditional third and fourth generation IOL formulas assume a fixed ratio between the curvature of the anterior and posterior surfaces of the cornea. Only a measurement of corneal anterior curvature is therefore required to compute corneal refractive power and IOL implant power after cataract surgery. However, in unusual eyes or eyes that have undergone refractive surgery, the relationship between the anterior and posterior corneal surfaces is likely to be altered and the assumptions of the traditional IOL calculation formulas become invalid.
Therefore, there is a need for methods and apparatus to perform biometry measurements.